Capacitance difference measuring circuit

ABSTRACT

A circuit and method for producing an output signal which is a function of the relative capacitances of two capacitors. Each capacitor is excited by an AC voltage. The voltage drop resultant on the charge passing through one capacitor is applied during only the positive half-cycle of the alternating voltage applied to that capacitor, across an output capacitor. The voltage developed by the charge passing through the other capacitor, during only the negative half-cycle of the alternating voltage applied thereto, is also applied across the output capacitor. The signal is observed by measuring the accumulated charge on the output capacitor, or the net charging current passing therethrough. The output signal is thus a plus or minus DC voltage and/or net charging current.

United States Patent [1 1 Machlanski Nov. 27, 1973 [76] Inventor: HenryT. Machlanski, Rd. 3,

Huntington, L. 1., N.Y. 11743 22 Filed: Dec. 17, 1971 [57] 21 Appl. No.:209,190

[52] US. Cl 324/60 C, 307/238, 324/59, 328/151 [51] Int. Cl. G01! 11/52,G01r 27/26 [58] Field of Search 324/60 C, 59, 111, 324/60 R; 307/257,238; 328/151 [56] References Cited UNITED STATES PATENTS 3,012,19212/1961 Lion 324/60 R X 3,577,072 5/1971 Miller 324/60 C 3,474,25910/1969 Rodgers 328/151 X 3,116,458 12/1963 Margopoulos 328/151 X3,077,544 2/1963 Connelly 1 307/257 3,201,641 8/1965 Thorne 307/257 X CXC! CAPACITANCE DIFFERENCE MEASURING CIRCUIT Primary ExaminerStanley T.Krawczewicz Attorney-John A. Reilly et a1.

ABSTRACT A circuit and method for producing an output signal which is afunction of the relative capacitances of two capacitors. Each capacitoris excited by an AC voltage. The voltage drop resultant on the chargepassing through one capacitor is applied during only the positivehalf-cycle of the alternating voltage applied to that capacitor, acrossan output capacitor. The voltage developed by the charge passing throughthe other capacitor, during only the negative half-cycle of thealternating voltage applied thereto, is also applied across the outputcapacitor. The signal is observed by measuring the accumulated charge onthe output capacitor, or the net charging current passing therethrough.The output signal is thus a plus or minus DC voltage and/or net chargingcurrent.

2 Claims, 6 Drawing Figures CAPACITANCE DIFFERENCE MEASURING CIRCUITBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to the field of electronic detection circuits and methods, andparticularly to circuits for producing a signal dependent on relativecapacitances.

2. Description of the Prior Art It is known to provide a detectorcircuit which provides a DC output varying in response to a change to achange in ratio of two capacitances. Such circuits typically include areference source of alternating voltage which is applied to acapacitance bridge and to a standard diode ring demodulator. Thecapacitive bridge contains, in two of its legs, the capacitors whosevarying relationship causes the change in output. Any change in theratio of these capacitances results in an output from the bridge whichis phase shifted with respect to the reference, the amount of shiftdepending on the ratio of capacitances. The output from the capacitancebridge is then applied to the ring demodulator. The diode ring comparesthe relative phase of the output of the capacitive bridge and that ofthe reference frequency input from the source, and furnishes a DC signalwhich is a function of that difference.

One disadvantage of such prior art circuits is that each requirescircuitry for providing a reference signal as well as a signal toactuate the bridge. Accordingly, it is one purpose of this invention toprovide a circuit for producing a signal dependent on relativecapacitances, wherein only one signal is used, i.e., the actuationsignal applied to the capacitors, and which has no need for a separatereference signal, and its associated circuitry.

The prior art circuitry operates, as noted above, on the basis of phaseshift between the output signals from the capacitive bridge and thereference signal. This dependence on phase shift to produce a signaloften imposes limits on the circuit designer in terms of the inductiveand reactive values of the other elements in associated circuitrybecause variations in such values will affect the crucial phaserelationship, within the typical capacitive detector device. It istherefore another ob ject of this invention to provide a capacitancedetector circuit which is totally independent of any phase relationshipsbetween signals in any portion of the circuit.

A source of some dissatisfaction in connection with the use of prior artcircuits is that the capacitive circuits employed typically have a highimpedance output. This generally means that the only practical use ofsuch an output is to measure its voltage. Such voltage measurementsacross high impedance outputs are susceptible to noise interference, andare therefore of limited utility. Current measurements of output,however, made across low-impedance output terminals are not sosusceptible to noise. Therefore, it is a further purpose of thisinvention to provide a circuit of the type described which is capable ofproviding an output which can be measured both as a high impedancevoltage output, and a low-impedance current output.

SUMMARY OF THE INVENTION This invention accomplishes the foregoing andother purposes by means of an apparatus including a voltage source forapplying AC excitation to first and second variable capacitors, at leastone being variable. An out put capacitor is also supplied, and circuitmeans is connected between the one plate of each of the capacitors, andthe output capacitor, for the purpose of charging the output capacitorto a degree which is dependent on the relative capacitances of thecapacitors.

The circuit means 'passes positive charge accumulating on the connectedplate of the first capacitor through a diode in a forward direction. Theresultant positive forward voltage across the diode is picked off andapplied to the output capacitor. The circuit means grounds directly anynegative charge on the connected plate of the first capacitor, bypassingthe output capacitor altogether.

Similarly, with respect to the second capacitor, the circuit passesnegative charge on the connected plate thereof backward through a diode,applying the negative voltage thus derived to the output capacitor.Positive charge on the connected plate is directly grounded.

The relative capacitances of the capacitors can then, therefore, bemeasured by simply ascertaining the DC accumulated net charge present onthe output capacitor. Moreover, the net charging current passing throughthe output capacitor can be measured by means of a low impedance device.Thus, this circuit is susceptible of application to both voltage andcurrent measurements, the output impedance being high in the case ofvoltage measurements, and low in the case of current measurements.

It can be seen that the circuitry of this invention does not requireseparate reference and excitation signals, the excitation signal on thecapacitor serving both to excite said capacitors and to provide theoutput signal. It is also clear that the circuitry of this invention isoperable independently of the relative phasing of the AC excitation ofthe two capacitors. The excitation need not be in phase for the circuitto operate properly.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of thedetector circuitry of this invention, showing the variable capacitorswhich are excited by an AC source, the diode circuitry, and itsconnection to the output capacitor.

FIG. 2(a) is a schematic diagram of that part of the circuitry whichfunctions when one of the variable capacitors is charged positively atits plate which is connected to the diode circuitry.

FIG. 2(b) is a schematic diagram of the functioning parts of thecircuitry when the same plate at said capacitor bears a negative charge.

FIG. 2(0) is a schematic diagram of the functioning portions of thediode circuitry when the other variable capacitor is charged negativelyat its plate which is connected to the diode circuitry.

FIG. 2(d) is a schematic diagram of the functioning portion of the diodecircuitry which is operable when said other capacitor bears a positivecharge on its plate which is connected to the diode circuitry.

FIG. 3 is a view showing the configuration of the excitation plate ofthe actuating capacitors.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a detailedschematic diagram of the preferred embodiment of the circuit of thisinvention. The capacitors which actuate the circuit include a commonexcitation plate C,, which is excited with an alternating voltage fromvoltage source 10. The capacitors also comprise capacitor plates C, andC as shown. C, is rotatably mounted on shaft 12. The configuration of Cmay be any configuration such that, as C, is rotated on shaft 12, therelative areas of capacitances C, and C can be changed. As an example C,may have a semicircular configuration, as shown in FIG. 3.

The output of the circuit of this invention appears across outputcapacitor C as shown, C being connected to each of plates C, and C bymeans of the diode circuitry indicated within zone 11, which isdiscussed hereinbelow.

Diode circuitry 11 is designed such that it applies positive charge tooutput capacitor C during that portion of the alternating voltage cyclein which positive voltage appears on capacitor plate C,. The amount ofcharge so deposited on capacitor C is a function of the capacitance ofthe capacitor comprising plate C, and that portion of excitation plateC, which is aligned proximate to plate C,. When a negative chargeappears on plate C,, that negative charge is drained off directly toground, and has virtually no effect on the charge present on outputcapacitor C In a converse manner, diode circuitry 11, in conjunctionwith plate C deposits negative charge on output capacitor C when plate Cbears negative charge, the amount of the charge so deposited being afunction of the capacitance of the capacitor formed by plate C and thatportion of plate C, which is aligned proximately to C This manner ofoperation is best described in connection with an examination of FIG.2(a)2(d). FIG. 2(a) illustrates the operative components of thecircuitry during that period in which a positive charge appears oncapacitor plates C, and C When plate C, accumulates positive charge byvirtue of the excitation of voltage source 10, that charge flows fromplate C, to ground through diodes 0,, D and D;,. It is noted at thispoint that this circuit operates and is effective by virtue of the factthat the diode is not in reality a perfect element, i.e., all diodeshave some forward voltage drop when positive charge is passed throughthem in a forward direction. Therefore, it can be seen that theillustrated connection between diodes D and D, and the upper plate ofoutput capacitor C will cause a positive charge to appear and to bedeposited on the upper plate of capacitor C while positive charge flowsfrom capacitor plate C, to ground through the three aforementioneddiodes. The amount of charge so deposited will be a function of thecharging voltage and the length of time during which charge flows fromC, to ground. Thus, since it is well known that capacitance, for a fixedvoltage applied, is proportional to the amount of charge carried by thecapacitor, the charge flowing through the three diodes and consequentlythe amount of positive charge deposited on capacitor C will be afunction of the capacitance of the capacitor comprising plate C, andthat portion of plate C, which is proximately aligned therewith. This,of course, assumes that C is sufficiently large to accumulate chargethereon during substantially all of the half-cycle of voltage applied.

Examining FIG. 2(d), it can be seen that the appearance of positivecharge on plate C concomitantly with the above described appearance ofpositive charge on C, has virtually no effect on output capacitor Csince any positive charge accumulating on C is drained off directly toground through diode D Considering the case in which plates C, and Cbear negative charge induced by alternating voltage source 10, FIG. 2(a)illustrates that negative charge on plate C passes therefrom to groundin a backward direction through diodes D,, D and D... This phenomenondue to the aforementioned voltage drop across the diodes, induces anegative voltage on capacitor C by virtue of the connection of the upperplate thereof to the point between diodes D, and D Similarly to themanner of operation described in FIG. 2(a) the amount of negative chargeso deposited on C will depend upon the length of time that negativecharge flows from C to ground in the above referenced manner. Therefore,the amount of negative charge deposited on C will be a function of thecapacitance of the capacitor comprising plate C and that portion ofplate C; which is aligned proximately therewith. With reference to FIG.2(b) the appearance of negative charge on capacitor plate C, hasvirtually no effect on output capacitor C such negative charge passingdirectly to ground through diode D,,, bypassing entirely the outputcapacitor C,,.

An interesting phenomenon with respect to capacitor C results from theoperation as described hereinabove. It will be noted that, if thecapacitances of the capacitors comprising C and C, and C respectively,are unequal, a net charge will appear across the plates of outputcapacitor C If the capacitance of the capacitor comprising plate C, isgreater than that of the capacitance comprising plate C the upper plateof capacitor C will have a net positive charge resident thereon (perhapswith a small ripple due to the alternating nature of the excitationvoltage). If, on the other hand, the capacitance of the capacitorcomprising plate C is greater than that of the plate C, capacitor, Cwill bear a net negative charge on its upper plate. Thus, it can be seenthat, with proper calibration, the ratio of capacitances between thecapacitors including plate C, and plate C, can be measured by means of ahigh impedance voltmeter placed across the terminals of capacitor Cillustrated at 22 in FIG. 1. It also follows that if the net accumulatedcharge across capacitor C is regarded as an output signal, this outputsignal can be changed within limits in a predictable fashion as afunction of the angular position of excitation plate C I with respect toplates C, and C Thus, this circuit is suitable either for measuring therelative capacitances of the capacitors comprising C, and C or for thegeneration of an output signal in response to the mechanical position ofexcitation plate C Such a property of this circuit has obvious utilityin connection with feedback control and servo systems.

It is also noteworthy that, in operation, a discernible net chargingcurrent will flow through capacitor C as a result of the alternatepositive and negative charge applied by virtue of the capacitors anddiode circuitry discussed hereinabove. It can be seen by inspection thatthis net charging current will be zero when the capacitances ofcapacitors comprising plates C, and C are equal, and will increase withincreasing inequality in these capacitances. Therefore, if the terminalsof output capacitor C are connected across the low input invertingterminals 18 of an operational amplifier 16, it is possible to generatea substantial signal at output 20 of the amplifier which is a functionof the amount of net charging current flowing through capacitor C Itcan, therefore, be seen that the circuitry of this invention is capableof producing an output which is susceptible of measurement either as ahigh impedance DC voltage output or as a low impedance DC currentoutput. As discussed hereinabove, prior circuits for accomplishingsimilar aims have been successful generally only as sources of highoutput impedance DC voltage signals.

lt is also noteworthy that the operation of this circuit is independentof the relative phase of the excitation signals impressed on plates C,and C Thus, plates C and C instead of being excited by a single voltagesource through single excitation plate C could be totally independentcapacitors, each excited by an entirely separate AC voltage source. Thecircuit would operate equally as well as in the described manner.Additionally, the magnitude of alternating voltage impressed on thecapacitor plates C and C need not even be equal, although inequality ofsuch voltage would result in the need for specialized calibration of thedc vice. Nonetheless, even if the voltages were unequal, the outputsignals on capacitor C would still vary as a function of the relativecapacitances of the capacitors comprising C and C Additionally, it is tobe noted that an effect equivalent to changing the relative capacitancesC and C can be obtained by varying the relative excitation voltages on Cand C In this way, the output signal can be made to vary as a functionof the voltages on one or both capacitors.

Although a number of diodes are suitable for application within thiscircuit, applicant has found that IN 9l4 diodes operate well. The diodesshould be chosen gen erally such that they have a relatively highforward voltage drop, in order to achieve the highest possible outputsignal on capacitor C The value of C must be sufficiently large tosmooth out the AC ripple from the excitation input somewhat, so that asuitable voltage measurement can be made across C Applicant has foundthat a suitable value for C lies in the range of 0001-01 microfarads. Asuitable frequency of operation for this circuit can be in the range of10,000 to 500,000 Hz.

Applicant has found that he can achieve with this circuitry, a DCvoltage output of up to approximately 0.8 volts. By applying the signalacross the terminals of capacitor C to the inverting low impedanceinputs of an operational amplifier, and thus measuring the currentthrough the capacitor, voltage signals of approximately 10 volts can beobtained.

The embodiment discussed herein employs variable capacitors to actuatethe circuit and produce the output signals. Applicant has also found,however, that, when the variable capacitors C and C are replaced byvariable inductors, the circuit operates in a manner analogous to thatalready described. The inductors are excited with an alternatingvoltage, and the voltage drop across each is a function if itsinductance. These voltage drops can be applied to the diode circuitrydescribed, just as the charge flow from the capacitor C, and C isapplied. This will induce current flow across the diodes, and alternatepositive and negative charging of the output capacitor. The charge, andnet charging current, on and through capacitor C will then be a functionof the relation values of the variable inductances.

It is to be noted that the embodiments discussed in this application areconsidered to be illustrative, rather than exhaustive, and that one canconstruct embodiments deviating from the specific embodiments shownherein without departing from the spirit of the invention. I

What is claimed is:

l. A method for providing a signal which is a function of the relativecharges generated on first and second capacitors, comprising the stepsof:

a. exciting said capacitors with alternating voltage,

b. applying only positive charge in a fixed polarity to an outputcapacitor by means of said first capacitor at a rate which issubstantially a function of the positive charge generated on one plateof said first capacitor, said step of applying positive chargecomprising:

i. passing forward through a first diode only positive charge generatedon one plate of said first capacitor and applying resultant positivevoltage drop across said diode to said output capacitor in a fixedpolarity, and

ii. passing the negative charge generated on said first capacitor toground by means of a diode, causing flow of said negative charge fromsaid first capacitor to bypass said first diode,

. applying only negative charge in a fixed polarity to said outputcapacitor by means of said second capacitor at a rate which issubstantially a function of the negative charge generated on one plateof said second capacitor, said step of applying negative chargecomprising:

'. passing in a backward direction through a second diode only negativecharge generated on one plate of said second capacitor, and applying theresultant negative voltage drop across said second diode to said outputcapacitor, in said fixed polarity, and

ii. passing positive charge generated on said second capacitor to groundby means of a diode causing flow of positive charge from said secondcapacitor to bypass said output capacitor, and

d. measuring one of the average net accumulated charge on said outputcapacitor and the value of net charging current through said capacitor.

2. An apparatus for providing a signal at an output element which is afunction of the relative charges on two capacitors, said apparatuscomprising:

a. first and second capacitors,

b. means connected to said capacitors to excite said capacitors withalternating voltage,

c. an output capacitor comprising said output element,

d. means connected between said first capacitor and said outputcapacitor for applying only positive charge by means of said firstcapacitor, and in fixed polarity to said output capacitor, the rate ofpositive charge so applied being substantially a function of thepositive charge generated on one plate of said first capacitor, saidmeans for applying positive charge comprising:

i. a first diode connected in a forward direction between said firstcapacitor and ground, and connections between said first diode and saidoutput capacitor to apply across said output capacitor the positivevoltage drop resultant on flow of positive charge from said firstcapacitor through said plying negative charge comprising: first diode,and i. a second diode connected in a backward direcii. diode meansconnected to said first capacitor to tion between said second capacitorand ground pass to ground only negative charge appearing on andconnections to apply across said output casaid first capacitor, theresultant charge flow bypacitor the voltage drop across said dioderesulpassing said connection to said output capacitor tant on flow ofnegative charge from said second e. means connected between said secondcapacitor capacitor through said second diode, and

and said output capacitor for applying only negaii. diode meansconnected to said second capacitor tive charge by means of said secondcapacitor and to pass to ground only positive charge appearing in afixed polarity to said output capacitor, the rate 10 on said secondcapacitor the resultant charge of negative charge so applied beingsubstantially a flow bypassing said connections to said output functionof the negative charge generated on one capacitor.

plate of said second capacitor, said means for ap-

1. A method for providing a signal which is a function of the relativecharges generated on first and second capacitors, comprising the stepsof: a. exciting said capacitors with alternating voltage, b. applyingonly positive charge in a fixed polarity to an output capacitor by meansof said first capacitor at a rate which is substantially a function ofthe positive charge generated on one plate of said first capacitor, saidstep of applying positive charge comprising: i. passing forward througha first diode only positive charge generated on one plate of said firstcapacitor and applying resultant positive voltage drop across said diodeto said output capacitor in a fixed polarity, and ii. passing thenegative charge generated on said first capacitor to ground by means ofa diode, causing flow of said negative charge from said first capacitorto bypass said first diode, c. applying only negative charge in a fixedpolarity to said output capacitor by means of said second capacitor at arate which is substantially a function of the negative charge generatedon one plate of said second capacitor, said step of applying negativecharge comprising: i. passing in a backward direction through a seconddiode only negative charge generated on one plate of said secondcapacitor, and applying the resultant negative voltage drop across saidsecond diode to said output capacitor, in said fixed polarity, and ii.passing positive charge generated on said second capacitor to ground bymeans of a diode causing flow of positive charge from said secondcapacitor to bypass said output capacitor, and d. measuring one of theaverage net accumulated charge on said output capacitor and the value ofnet charging current through said capacitor.
 2. An apparatus forproviding a signal at an output element which is a function of therelative charges on two capacitors, said apparatus comprising: a. firstand second capacitors, b. means connected to said capacitors to excitesaid capacitors with alternating voltage, c. an output capacitorcomprising said output element, d. means connected between said firstcapacitor and said output capacitor for applying only positive charge bymeans of said first capacitor, and in fixed polarity to said outputcapacitor, the rate of positive charge so applied being substantially afunction of the positive charge generated on one plate of said firstcapacitor, said means for applying positive charge comprising: i. afirst diode connected in a forward direction between said firstcapacitor and ground, and connections between said first diode and saidoutput capacitor to apply across said output capacitor the positivevoltage drop resultant on flow of positive charge from said firstcapacitor through said first diode, and ii. diode means connected tosaid first capacitor to pass to ground only negative charge appearing onsaid first capacitor, the resultant charge flow bypassing saidconnection to said output capacitor, e. means connected between saidsecond capacitor and said output capacitor for applying only negativecharge by means of said second capacitor and in a fixed polarity to saidoutput capacitor, the rate of negative charge so applied beingsubstantially a function of the negative charge generated on one plateof said second capacitor, said means for applying negative chargecomprising: i. a second diode connected in a backward direction betweensaid second capacitor and ground and connections to apply across saidoutput capacitor the voltage drop across said diode resultant on flow ofnegative charge from said second capacitor through said second diode,and ii. diode means connected to said second capacitor to pass to groundonly positive charge appearing on said second capacitor the resultantcharge flow bypassing said connections to said output capacitor.